Heretofore, a variety of plants have traditionally been cultivated outdoors, i.e. by field cultivation utilizing the blessings of nature such as sunlight and soil. On the other hand, protected (or house) cultivation, i.e. cultivation in greenhouses or hothouses covered with glass or polymer films, has come to be widely conducted recently. Such protected cultivation is less liable to be affected by changes in environmental conditions than the field cultivation, and therefore it has an advantage of enabling stable production of various plants (for example vegetables).
According to statistics in 1999, the total area of protected cultivation of vegetables has amounted to 112,822 ha, which indicates the great contribution of protected cultivation in that it has enabled the year-round supply of various vegetables by eliminating no-harvest seasons despite a certain criticism of inferior nutritional value of the products compared to those produced by field cultivation.
In the protected cultivation of plants, the so-called nutrient fluid cultivation (generally referred to as “hydroponic cultivation” in most cases) is beginning to be introduced wherein plants are cultivated by using equipment in greenhouses, without using soil to maintain them, and supplying nutrient and water to them, from viewpoints that it can avoid damages caused by repeated cultivation, it permits cultivation at areas not suitable for cultivation, it leads to reduced working hours and increased yields per unit area compared to the field cultivation, and it has a potential applicability for industrial production. As compared to the cultivation using soil, this nutrient fluid cultivation has advantages that there are no damages caused by repeated cultivation (this merit is particularly great since a total ban on using methyl bromide for use in fumigation is to be implemented in the near future), the growth is generally rapid with high yields, and it is relatively easy to regulate the cultivation environment, etc.
However, the area for nutrient fluid cultivation in 1999 is 1056 ha, which is merely about 1% of the total area for protected cultivation of vegetables. Major reasons for such a lack of growth of nutrient fluid cultivation include: the fact that an initial capital investment is large, the production costs becomes high leading to an increased risk, the operation requires considerable skills, and a failure, even once, in cultivation (contamination by pathogens, the maladjustment of nutrient fluid, etc.) may cause a devastating damage, etc.
Types of nutrient fluid cultivation are classified into three kinds, including: mist cultivation, hydroponic cultivation (deep flow technique, NFT), and solid medium cultivation (sand cultivation, gravel cultivation, rock fiber cultivation). These types of the cultivation have their respective advantages and disadvantages (with respect to details and merits and demerits of each of these types, the paper “New Manual for Nutrient Fluid Cultivations” (YOUEKI SAIBAI NO SHIN MANYUARU) edited by the Japan Greenhouse Horticulture Association, issued by Seibundo Shinkosha Inc., in July, 2002 may be referred to).
The most important points or drawbacks common to each of the above-mentioned nutrient fluid cultivation systems are high initial costs, high running costs, and difficulty in oxygen supply. Further, since the root and the nutrient fluid come into direct contact with each other in the nutrient fluid cultivation, the nutrient fluid needs to be delicately controlled and the tolerable control range is very narrow, and such requirements pose a problem. In particular, utmost care must be taken on changes in the composition, concentration, and pH of the nutrient fluid. Among them, one of the most serious problems in the nutrient fluid cultivation is that the pH of the nutrient fluid may change very easily.
The supply of oxygen to plants is also the most important condition. In particular, there is a problem of oxygen deficiency, because the oxygen demand is increased high at a high temperature due to the enhanced respiration of the root, while the concentration of dissolved oxygen in the nutrient fluid becomes reduced at a high temperature. When the oxygen shortage occurs, a phenomenon of so-called “suffocation of root” arises with a result that the root become decayed, ammonia is produced, and the pH of the nutrient fluid starts to rise. In the nutrient fluid cultivation, the oxygen supply to the root is conducted by dissolving oxygen in the fluid or by the exposure of the root to the air for certain period. However, in general, the use of the dissolved oxygen is inevitable in the nutrient fluid, because of the system configuration in the nutrient fluid cultivation. However, because of the property of the poor solubility of oxygen in the nutrient fluid, it is impossible to increase the concentration of the dissolved oxygen up to adequate level, and in an actual example of the nutrient fluid cultivation, the supply of oxygen to plants is inadequate in a very large number of cases thereof.
In addition, the prevention of infection by pathogenic microorganisms is an extremely serious problem in a very large number of cases in the conventional nutrient fluid cultivation. In order to prevent the infection, various efforts have been made. Although the administration of agricultural chemicals may be contemplated, they cannot be added to the culture liquid because of their registration as agricultural chemicals, and thus various bactericidal methods that do not depend on agricultural chemicals have been devised. Specific examples thereof include: the sterilization with UV, ozone, heat, etc., the elimination of pathogenic microorganisms by filtration, the sterilization by the addition of a metal ion such as silver, and: the addition of antagonistic microorganisms, etc. However, all of them pose the problem of increased costs due to the installment and control of additional equipment, and further they may damage plants or pose new problems of, for example, decomposing an active component in the nutrient fluid, and therefore no definitive effects of preventing the infection have been obtained.
(Non-Patent Document 1)
“New Manual for Nutrient Fluid Cultivations” (YOUEKI SAIBAI NO SHIN MANYUARU) edited by the Japan Greenhouse Horticulture Association, issued by Seibundo Shinkosha Inc. in July, 2002